envknob: add GOOS func

Centralize the fake GOOS stuff, start to use it more. To be used more
in the future.

Change-Id: Iabacfbeaf5fca0b53bf4d5dbcdc0367f05a205f9
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/tailscaled.go

@@ -274,12 +274,7 @@ func statePathOrDefault() string {
 }
 
 func ipnServerOpts() (o ipnserver.Options) {
-	// Allow changing the OS-specific IPN behavior for tests
-	// so we can e.g. test Windows-specific behaviors on Linux.
-	goos := envknob.String("TS_DEBUG_TAILSCALED_IPN_GOOS")
-	if goos == "" {
-		goos = runtime.GOOS
-	}
+	goos := envknob.GOOS()
 
 	o.VarRoot = args.statedir
 

envknob/envknob.go

@@ -277,11 +277,9 @@ func SSHPolicyFile() string { return String("TS_DEBUG_SSH_POLICY_FILE") }
 // SSHIgnoreTailnetPolicy is whether to ignore the Tailnet SSH policy for development.
 func SSHIgnoreTailnetPolicy() bool { return Bool("TS_DEBUG_SSH_IGNORE_TAILNET_POLICY") }
 
-
 // TKASkipSignatureCheck is whether to skip node-key signature checking for development.
 func TKASkipSignatureCheck() bool { return Bool("TS_UNSAFE_SKIP_NKS_VERIFICATION") }
 
-
 // NoLogsNoSupport reports whether the client's opted out of log uploads and
 // technical support.
 func NoLogsNoSupport() bool {

envknob/envknob_nottest.go

@@ -0,0 +1,17 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ts_not_in_tests
+
+package envknob
+
+import "runtime"
+
+func GOOS() string {
+	// When the "ts_not_in_tests" build tag is used, we define this func to just
+	// return a simple constant so callers optimize just as if the knob were not
+	// present. We can then build production/optimized builds with the
+	// "ts_not_in_tests" build tag.
+	return runtime.GOOS
+}

envknob/envknob_testable.go

@@ -0,0 +1,24 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !ts_not_in_tests
+
+package envknob
+
+import "runtime"
+
+// GOOS reports the effective runtime.GOOS to run as.
+//
+// In practice this returns just runtime.GOOS, unless overridden by
+// test TS_DEBUG_FAKE_GOOS.
+//
+// This allows changing OS-specific stuff like the IPN server behavior
+// for tests so we can e.g. test Windows-specific behaviors on Linux.
+// This isn't universally used.
+func GOOS() string {
+	if v := String("TS_DEBUG_FAKE_GOOS"); v != "" {
+		return v
+	}
+	return runtime.GOOS
+}

ipn/ipnlocal/profiles.go

@@ -13,6 +13,7 @@ import (
 	"time"
 
 	"golang.org/x/exp/slices"
+	"tailscale.com/envknob"
 	"tailscale.com/ipn"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/logger"
@@ -423,7 +424,7 @@ func ReadStartupPrefsForTest(logf logger.Logf, store ipn.StateStore) (ipn.PrefsV
 // It also loads the list of known profiles from the StateStore.
 // If a state key is provided, it will be used to load the current profile.
 func newProfileManager(store ipn.StateStore, logf logger.Logf, stateKey ipn.StateKey) (*profileManager, error) {
-	return newProfileManagerWithGOOS(store, logf, stateKey, runtime.GOOS)
+	return newProfileManagerWithGOOS(store, logf, stateKey, envknob.GOOS())
 }
 
 func readAutoStartKey(store ipn.StateStore, goos string) (ipn.StateKey, error) {

tstest/integration/integration_test.go

@@ -809,7 +809,7 @@ func (n *testNode) StartDaemonAsIPNGOOS(ipnGOOS string) *Daemon {
 		"TS_LOG_TARGET="+n.env.LogCatcherServer.URL,
 		"HTTP_PROXY="+n.env.TrafficTrapServer.URL,
 		"HTTPS_PROXY="+n.env.TrafficTrapServer.URL,
-		"TS_DEBUG_TAILSCALED_IPN_GOOS="+ipnGOOS,
+		"TS_DEBUG_FAKE_GOOS="+ipnGOOS,
 		"TS_LOGS_DIR="+t.TempDir(),
 		"TS_NETCHECK_GENERATE_204_URL="+n.env.ControlServer.URL+"/generate_204",
 	)